It was good to talk.
You need to write to them (email is fine) telling them that you have now taken advice in respect of this and that they have until (say) 4 PM on Friday, 18 June 2021 to come up with a satisfactory proposal to reimburse you in full and collect the item within the fortnight thereafter failing which you will issue Small Claims Court proceedings without further notice on the basis that they are:
In breach of the provisions of the Consumer Rights Act 2015 in that the item is
not as described
Of unsatisfactory quality.
Whilst you accept that there may be variations in the grey you do not accept that the variations in the grey would amount to brown. If you had wanted grey and brown you would have asked for it. Further, whilst the image on the website does show variations in the colour of the concrete, it does not show any brown.
They are unlikely to agree to take it back in which case you are going to end up issuing Small Claims Court proceedings.
The new Moneyclaim beta system is easy to use: https://www.gov.uk/make-money-claim
the new system is much better than the old government Gateway system.
You can prepare the proceedings now, at your leisure, save them, and then just pay the fee and press the button when the deadline in your letter before action, expires.
It's then just a case of following the court process and responding to any emails that you receive.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have